Cutting insert, cutting tool, and method for manufacturing cut workpiece using same

ABSTRACT

A cutting insert, a cutting tool and method of manufacturing a machined product. The cutting insert includes: upper and lower surfaces; a side surface; a cutting edge; and a rake portion on the upper surface. The cutting edge includes major and minor cutting edges that are convex toward outside of the main body portion. The minor cutting edge has a curvature radius smaller than a curvature radius of the major cutting edge. The rake portion is inclined and approaches the lower surface as moving from the major cutting edge toward inside of the main body portion and is located along the major cutting edge. An inclination angle of the rake portion becomes smaller as moving from a part continuous with one end of the major cutting edge toward a part continuous with center of the major cutting edge.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase of the International applicationPCT/JP2012/066578 filed Jun. 28, 2012 that claims priority from theJapanese patent application JP2011-145347 filed Jun. 30, 2011. Thecontent of these aforementioned documents is herewith incorporated byreference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a cutting insert, a cutting tool, and amethod of manufacturing a cut workpiece using same.

BACKGROUND ART OF THE INVENTION

In machining of metal materials so-called difficult-to-cut materialssuch as heat resistant alloys and high hardness materials (hereinaftersimply referred to as machining of difficult-to-cut materials), acutting insert having a cutting edge that is arcuate in plan view, suchas that disclosed in International Publication No. 2010/023659, is used.Such a cutting insert is advantageous in machining of difficult-to-cutmaterials compared to the case of a cutting edge that is linear in planview, from the viewpoint that generated chips are thin and that thecutting edge strength is high.

However, generated chips become thicker with the increase in depth ofcut, and therefore, even when the cutting insert described above isused, cutting resistance may increase and the cutting edge may becomeliable to fracture.

PTL 1: International Publication No. 2010/023659 SUMMARY OF INVENTION

A cutting insert according to an embodiment of the present inventionincludes a main body portion that includes: an upper surface; a lowersurface; a side surface connected to the upper surface and the lowersurface; and a cutting edge located at the intersection of the uppersurface and the side surface. The cutting edge includes: an arcuatemajor cutting edge that is convex toward outside of the main bodyportion; and an arcuate minor cutting edge that is convex toward theoutside of the main body portion and that has a curvature radius smallerthan a curvature radius of the arcuate major cutting edge. The uppersurface includes a first rake portion that is inclined so as to approachthe lower surface as moving from the arcuate major cutting edge towardinside of the main body portion and that is provided along the arcuatemajor cutting edge. An inclination angle of the first rake portionbecomes smaller as moving from a part continuous with one end of thearcuate major cutting edge toward a part continuous with center of thearcuate major cutting edge.

A cutting tool according to an embodiment of the present inventionincludes the cutting insert according to the above-described embodiment,and a cylindrical holder having a rotation center axis. The cuttinginsert is attached to the holder such that the arcuate major cuttingedge protrudes from the outer peripheral surface of the holder.

A method of manufacturing a machined product of the present inventionincludes steps of: rotating the cutting tool according to theabove-described embodiment; bringing the cutting edge of the rotatingcutting tool into contact with a workpiece and cutting the workpiece;and separating the cutting edge of the rotating cutting tool from theworkpiece.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view showing an example of an embodiment of acutting insert of the present invention.

FIG. 2 is a plan view of the cutting insert shown in FIG. 1.

FIG. 3 (a) is a side view of the cutting insert shown in FIG. 2 as seenfrom the direction of arrow A. FIG. 3 (b) is a side view of the cuttinginsert shown in FIG. 2 as seen from the direction of arrow B.

FIG. 4 is a sectional view of the cutting insert shown in FIG. 2 takenalong line C-C.

FIG. 5 is a sectional view of the cutting insert shown in FIG. 2 takenalong line D-D.

FIG. 6 is a sectional view of the cutting insert shown in FIG. 2 takenalong line E-E.

FIG. 7 is a sectional view of the cutting insert shown in FIG. 2 takenalong line F-F.

FIG. 8 is a sectional view of the cutting insert shown in FIG. 2 takenalong line G-G.

FIG. 9 is a sectional view of the cutting insert shown in FIG. 2 takenalong line H-H.

FIG. 10 is a sectional view of the cutting insert shown in FIG. 3 takenalong line I-I.

FIG. 11 is a bottom view of the cutting insert shown in FIG. 1.

FIG. 12 is a perspective view showing an example of an embodiment of acutting holder of the present invention.

FIG. 13 (a) is a partial enlarged perspective view of the region J shownin FIG. 12. FIG. 13 (b) is a plan view of the region shown in FIG. 13(a).

FIG. 14 (a) is a perspective view showing an example of an embodiment ofa cutting tool of the present invention. FIG. 14 (b) is a side view ofthe cutting tool shown in FIG. 14 (a). FIG. 14 (c) is a partial enlargedview of the region K shown in FIG. 14 (a).

FIG. 15 is a process flow diagram illustrating an example of anembodiment of a method of cutting a workpiece of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS <Cutting Insert>

A cutting insert 1 (hereinafter simply referred to as insert 1) that isan example of an embodiment of the present invention will be describedbelow with reference to FIG. 1 to FIG. 11.

The insert 1 that is an example of an embodiment of the presentinvention includes a main body portion 10 having an upper surface 2, alower surface 3, and a side surface 4 connected to the upper surface 2and the lower surface 3. In this example, as shown in FIG. 1 and FIG. 2,the main body portion 10 is plate-like. The shape of the main bodyportion 10 is, in plan view, for example, a circular or polygonal suchas triangular, quadrangular, pentagonal, hexagonal, or octagonal shape.These shapes are shapes that those skilled in the art usually use for aninsert. In this example, specifically, the main body portion 10 issubstantially circular. In this example, the dimensions of the main bodyportion 10 is such that the maximum width of the upper surface 2 is 5 mmto 20 mm and the height from the lower surface 3 to the upper surface 2is 2 mm to 8 mm. A cutting edge 5 is located at the intersection of theupper surface 2 and the side surface 4.

Examples of the material of the insert 1 include cemented carbide andcermet. Examples of the composition of the cemented carbide includeWC—Co produced by adding powder of cobalt (Co) to tungsten carbide (WC)followed by sintering, WC—TiC—Co obtained by adding titanium carbide(TiC) to WC—Co, and WC—TiC—TaC—Co obtained by adding tantalum carbide(TaC) to WC—TiC—Co. The cermet is a sintered composite material obtainedby combining a ceramic component with metal, and specific examplesthereof are titanium compounds composed mainly of titanium carbide (TiC)or titanium nitride (TiN).

The surface of the insert 1 may be coated using a chemical vapordeposition (CVD) method or a physical vapor deposition (PVD) method.Examples of the composition of the coating include titanium carbide(TiC), titanium nitride (TiN), titanium carbonitride (TiCN), or alumina(Al₂O₃).

A region of the upper surface 2 along the cutting edge 5 functions as arake surface scraped by chips. A region of the side surface 4 along thecutting edge 5 functions as a flank surface.

In plan view, the cutting edge 5 includes arcuate major cutting edges 51that are convex toward the outside of the main body portion 10 and thathave a curvature radius r1, and arcuate minor cutting edges 52 that areconvex toward the outside of the main body portion 10 and that have acurvature radius r2. The curvature radius r2 of the arcuate minorcutting edges 52 is smaller than the curvature radius r1 of the arcuatemajor cutting edges 51. That is, r1>r2. Specifically, in this example,the curvature radius of the arcuate major cutting edges 51 is 6.0 mm,and the curvature radius of the arcuate minor cutting edges 52 is 4.5mm. As described above, the insert 1 has a cutting blade 5 having aplurality of curvature radii, and therefore the formation of a complexcurved surface, such as three-dimensional machining, can be easilyperformed.

In this example, as shown in FIG. 2, in plan view, the length of thearcuate major cutting edges 51 is larger than the length of the arcuateminor cutting edges 52. With this configuration, the arcuate majorcutting edges 51 functioning as major cutting edges can be secured to belonger than the arcuate minor cutting edges 52 functioning as minorcutting edges.

As shown in FIG. 3, the arcuate major cutting edges 51 are inclined soas to approach the lower surface 3 with increasing distance from thearcuate minor cutting edges 52. The arcuate minor cutting edges 52 areinclined so as to approach the lower surface 3 with increasing distancefrom the arcuate major cutting edges 51. With this configuration, thearea of contact with a workpiece is reduced, and cutting resistance isreduced.

Therefore, relative vibration (hereinafter referred to as “chattervibration”) caused between the arcuate major cutting edges 51 and thearcuate minor cutting edges 52 and the workpiece can be prevented. As aresult, the insert 1 is suitable for cutting requiring machined surfaceaccuracy, such as profile machining. In addition, heat generation at thetime of cutting is prevented by the reduction in contact area, and theinsert 1 is suitable for the machining of difficult-to-cut materialssuch as heat resistant alloys and high hardness materials.

As shown in FIG. 3, the highest points 53 of the cutting edge 5 that isfarthest from the lower surface 3 are located between the arcuate majorcutting edge 51 and the arcuate minor cutting edge 52. Specifically, thehighest points 53 farthest from the lower surface 3 are located betweenthe higher end of the arcuate major cutting edge 51 and the higher endof the arcuate minor cutting edge 52. That is, the higher end of thearcuate major cutting edge 51 and the higher end of the arcuate minorcutting edge 52 are located at the same height from the lower surface 3.With this configuration, the highest points 53 function as cornerportions between the arcuate major cutting edge 51 and the arcuate minorcutting edge 52 and easily engage into the workpiece, cutting resistanceis reduced, and chatter vibration can be prevented.

As shown in FIG. 3, the lowest points 54 of the cutting edge 5 nearestto the lower surface 3 are located at the lower end of each arcuatemajor cutting edge 51 and the lower end of each arcuate minor cuttingedge 52. That is, the lower end of each arcuate major cutting edge 51and the lower end of each arcuate minor cutting edge 52 are located atthe same height from the lower surface 3. With this configuration, themaximum depth of cut of the arcuate major cutting edges 51 is equal tothe maximum depth of cut of the arcuate minor cutting edge 52.

In this example, as shown in FIG. 3 (a), in side view, the arcuate majorcutting edges 51 and the arcuate minor cutting edges 52 are curvedtoward a side away from the lower surface 3. With this configuration,thrust force can be well distributed, and fracture of the arcuate majorcutting edges 51 and the arcuate minor cutting edges 52 can beprevented. Although, in this example, they are formed of a curved linehaving a curvature radius of 15 mm, they may be formed of a plurality ofcurved lines having a plurality of curvature radii.

As shown in FIG. 1 and FIG. 2, the upper surface 2 has a rake surface 20that is continuous with the cutting edge 5 and that is inclined so as toapproach the lower surface 3 as moving from the cutting edge 5 towardthe inside of the main body portion. The rake surface 20 is involved inthe thickness of chips generated by the cutting edge 5. In general, thethickness of chips tends to decreases with increasing rake angle. Sincethe upper surface 2 has the rake surface 20 continuous with the cuttingedge 5, the engagement of the cutting edge 5 into the workpiece is good,and cutting resistance can be reduced.

In this example, as shown in FIG. 1 and FIG. 2, in order to enhance thestrength of the cutting edge 5, a flat land 6 is provided at theintersection of the cutting edge 5 and the rake surface 20. The width ofthe land 6 is appropriately set in accordance with cutting conditions,and is 0.12 mm in this example. In this example, since the cutting edge5 is arcuate, chips generated by the cutting edge 5 are thin compared toa linear cutting edge. Therefore, chips are easily spirally curled byscraping the rake surface 20. That is, the chip ejection direction iscontrolled by the rake surface 20. If it is difficult to control thechip ejection direction owing to cutting conditions, it is recommendedto separately provide a recess or protrusion on the upper surface 2 andto appropriately design the breaker shape.

The upper surface 2 includes first rake portions 21 that are inclined soas to approach the lower surface 3 as moving from the arcuate majorcutting edges 51 toward the inside of the main body portion 10 and thatare provided along the arcuate major cutting edges 51. The inclinationangle of the first rake portions 21 becomes smaller as moving from apart continuous with one end of the arcuate major cutting edge 51 towarda part continuous with the center of the arcuate major cutting edge 51.With this configuration, the cutting edge strength of each arcuate majorcutting edge 51 becomes stronger as moving from one end of each arcuatemajor cutting edge 51 toward the center thereof. Therefore, the fractureof the arcuate major cutting edges 51 can be prevented even in machiningin which the depth of cut of the arcuate major cutting edges 51increases.

In particular, when, like the first rake portions in this example, theinclination angle becomes smaller as moving from a part continuous withone end of the arcuate major cutting edge toward a part continuous withthe other end of the arcuate major cutting edge, the fracture of thearcuate major cutting edges 51 can be prevented even in machining inwhich the depth of cut of the arcuate major cutting edges 51 furtherincreases.

FIG. 4 shows a cross section in the normal direction of one end of oneof the arcuate major cutting edges 51. FIG. 5 shows a cross section inthe normal direction of the center of the arcuate major cutting edge 51.FIG. 6 shows a cross section in the normal direction of the other end ofthe arcuate major cutting edge 51.

In this example, specifically, the angle between a virtual extensionline P1 of the end of the first rake portion 21 shown in FIG. 4 nearerto the arcuate major cutting edge 51 and a reference plane h1 parallelto a horizontal plane passing through the arcuate major cutting edge 51is denoted as α1. The angle between a virtual extension line P2 of theend of the first rake portion 21 shown in FIG. 5 nearer to the arcuatemajor cutting edge 51 and the reference plane h1 parallel to ahorizontal plane passing through the arcuate major cutting edge 51 isdenoted as α2. The angle between a virtual extension line P3 of the endof the first rake portion 21 shown in FIG. 6 nearer to the arcuate majorcutting edge 51 and the reference plane h1 parallel to a horizontalplane passing through the arcuate major cutting edge 51 is denoted asα3.

At this time, α1>α2>α3. The angles α1, α2, α3 are appropriately set fromthe range of 10° to 40°. In this example, specifically, α1=32°, α2=25°,and α3=21°. Since FIG. 4 to FIG. 6 are all side views, the horizontalplane h1 is represented by a straight line.

In plan view, the first rake portions 21 are nearest to the inside ofthe main body portion (rotation center axis S1) in a regioncorresponding to the center of the arcuate major cutting edge 51, andare farthest from the inside of the main body portion (rotation centeraxis S1) in regions corresponding to both ends of the arcuate majorcutting edge 51. Specifically, in this example, as shown in FIG. 4 toFIG. 6, the length w1 in the normal direction of the arcuate majorcutting edge 51 is largest in a region corresponding to the center ofthe arcuate major cutting edge 51, and is smallest in regionscorresponding to both ends of the arcuate major cutting edge 51. Withsuch a configuration, the contact area between generated chips and theupper surface 2 is reduced, and the reduction in cutting resistance canbe promoted. As a result, the fracture of the arcuate major cuttingedges 51 is further prevented.

The upper surface 2 has, along the arcuate minor cutting edges 52,second rake portions 22 that are inclined so as to approach the lowersurface 3 as moving from the arcuate minor cutting edges 52 toward theinside of the main body portion. The inclination angle of the secondrake portions 22 becomes smaller as moving from a part continuous withone end of the arcuate minor cutting edge 52 toward a part continuouswith the center of the arcuate minor cutting edge 52. With thisconfiguration, the cutting edge strength of each arcuate minor cuttingedge 52 becomes stronger as moving from one end of each arcuate minorcutting edge 52 toward the center thereof. Therefore, the fracture ofthe arcuate minor cutting edges 52 can be prevented even in machining inwhich the depth of cut of the arcuate minor cutting edges 52 increases.

FIG. 7 shows a cross section in the normal direction of one end of oneof the arcuate minor cutting edges 52. FIG. 8 shows a cross section inthe normal direction of the center of the arcuate minor cutting edge 52.FIG. 9 shows a cross section in the normal direction of the other end ofthe arcuate minor cutting edge 52.

In this example, specifically, the angle between a virtual extensionline Q1 of the end of the second rake portion 22 shown in FIG. 7 nearerto the arcuate minor cutting edge 52 and a reference plane h2 parallelto a horizontal plane passing through the arcuate minor cutting edge 52is denoted as β1. The angle between a virtual extension line Q2 of theend of the second rake portion 22 shown in FIG. 8 nearer to the arcuateminor cutting edge 52 and the reference plane h2 parallel to ahorizontal plane passing through the arcuate minor cutting edge 52 isdenoted as β2. The angle between a virtual extension line Q3 of the endof the second rake portion 22 shown in FIG. 9 nearer to the arcuateminor cutting edge 52 and the reference plane h2 parallel to ahorizontal plane passing through the arcuate minor cutting edge 52 isdenoted as β3.

At this time, β1<β2<β3. The angles β1, β2, and β3 are appropriately setfrom the range of 10° to 40°. Since FIG. 7 to FIG. 9 are all side views,the horizontal plane h2 is represented by a straight line.

In plan view, the width of the second rake portions 22 from the arcuateminor cutting edges 52 to the inside of the main body portion issubstantially constant. With this configuration, the stability of thedirection in which generated chips are ejected can be improved. In thisexample, the width of the second rake portions 22 from the arcuate minorcutting edge 52 to the inside of the main body portion means the lengthin the normal direction of the arcuate minor cutting edge 52. The abovewidth is specifically w2 shown in FIG. 7 to FIG. 9. It is clear that thewidth w2 of the second rake parts 22 is substantially constant over theentire length of the second rake portions 22. The term “substantiallyconstant” is defined to mean that the variation in size is ±2 mm.

In this example, as shown in FIG. 1 and FIG. 2, the main body portion 10has, in its center, a through-hole 7 penetrating the upper surface 2 andthe lower surface 3. The cutting edge 5 has a plurality of cuttingportions 50 each of which includes the arcuate major cutting edge 51 andthe arcuate minor cutting edge 52 around the through-hole 7. Thethrough-hole 7 is a hole for inserting an attaching screw for attachingto a holder (to be described later). With this configuration, the insert1 can be rotated about the through hole 7, each of the plurality ofcutting portions can be used for cutting, and economic efficiency isimproved.

Specifically, as shown in FIG. 2, three cutting portions 50 (50 a, 50 b,50 c) are formed around the through-hole 7. Each cutting portion has onearcuate major cutting edge 51 and one arcuate minor cutting edge 52.That is, the cutting edge 5 has three arcuate major cutting edges 51 (51a, 51 b, 51 c) and three arcuate minor cutting edges 52 (52 a, 52 b, 52c). In this example, in plan view, arcuate major cutting edges 51 a, 51b, and 51 c and arcuate minor cutting edges 52 a, 52 b, and 52 c arealternately arranged in the order of the arcuate major cutting edge 51a, the highest point 53, the arcuate minor cutting edge 52 a, the lowestpoint 54, the arcuate major cutting edge 51 b, . . .

Here, the shape formed by alternately arranging the arcuate majorcutting edges 51 and the arcuate minor cutting edges 52 is notparticularly limited. For example, in the case where the shape of themain body portion 10 is a polygonal shape in plan view, the polygonalshape may be formed by disposing an arcuate major cutting edge 51 oneach side of the upper surface 2 and disposing an arcuate minor cuttingedge 52 on each corner of the upper surface 2.

In this example, the upper surface 2 is substantially triangular asshown in FIG. 2, and has three arcuate major cutting edges 51 (51 a, 51b, 51 c) on three sides and three arcuate minor cutting edges 52 (52 a,52 b, 52 c) on three corners. By arranging the arcuate major cuttingedges 51 and arcuate minor cutting edges 52 in a polygonal shape in planview as described above, the increase in size of the insert 1 caused bythe increase in curvature radii of arcuate major cutting edges 51 andarcuate minor cutting edges 52 can be prevented.

The shape of the cutting edge 5 may be different from theabove-described shape. The cutting edge 5 in this example has aplurality of arcuate major cutting edges 51 and a plurality of arcuateminor cutting edges 52, and one arcuate minor cutting edge 52 isdisposed between each adjacent two of the arcuate major cutting edges51. As described above, the cutting edge in this example is formed onlyof arcuate major cutting edges 51 and arcuate minor cutting edges 52.However, the configuration of the cutting edge is not limited to such aconfiguration. For example, the cutting edge may be configured such thatit has a linear cutting edge between an arcuate major cutting edge 51and an arcuate minor cutting edge 52.

Alternatively, the cutting edge may be configured such that a linearcutting edge is located between each adjacent two of a plurality ofarcuate major cutting edges 51, and an arcuate minor cutting edge 52 islocated between each arcuate major cutting edge 51 and the linearcutting edge. In such a case, the cutting edges 5 are arranged in theorder of arcuate major cutting edge 51, arcuate minor cutting edge 52,linear cutting edge, arcuate minor cutting edge 52, arcuate majorcutting edge 51, . . . The above “arcuate” and “linear” mean shapes whenviewed from above. Therefore, it is not intended to require that they be“arcuate” and “linear” even when they are viewed from the side.

As shown in FIG. 2, the upper surface 2 has a flat surface 23 locatednearer to the inside of the main body portion 10 than the first rakeportions 21 and the second rake portions 22. The flat surface 23 isperpendicular to the rotation center axis S1. The flat surface 23 islower than the first rake portions 21 and the second rake portions 22.In plan view, the flat surface 23 is farthest from the arcuate majorcutting edges 51 in the center of each arcuate major cutting edge 51.With this configuration, the cutting edge strength of the center of eacharcuate major cutting edge 51 can be improved, and chatter vibration canbe prevented.

As shown in FIG. 2, the upper surface 2 has a stepped portion 24 that islocated between the flat surface 23 and the first rake portions 21 andthe second rake portions 22. The stepped portion 24 is inclined so as toapproach the lower surface 3 toward the inside of the main body portion.The inclination angle of the stepped portion 24 becomes smaller withincreasing distance from the arcuate minor cutting edges 52. Further, inthe regions corresponding to the centers of the plurality of arcuatemajor cutting edges 51, the inclination angle of the stepped portion 24is equal to the inclination angle of the first rake portions 21. Thearcuate major cutting edges 51 are used as major cutting edges. In thecenter of each arcuate major cutting edge 51, a large region functioningas a rake in the upper surface 2 can be secured. Therefore, thestability of the direction in which generated chips are ejected can beimproved.

In this example, specifically, the angle between a virtual extensionline R1 of the end of the stepped portion 24 shown in FIG. 4 nearer tothe arcuate major cutting edge 51 and the reference plane h1 parallel toa horizontal plane passing through the arcuate major cutting edge 51 isdenoted as γ1. The angle between a virtual extension line R2 of the endof the stepped portion 24 shown in FIG. 5 nearer to the arcuate majorcutting edge 51 and the reference plane h1 parallel to a horizontalplane passing through the arcuate major cutting edge 51 is denoted asγ2. The angle between a virtual extension line R3 of the end of thestepped portion 24 shown in FIG. 6 nearer to the arcuate major cuttingedge 51 and the reference plane h1 parallel to a horizontal planepassing through the arcuate major cutting edge 51 is denoted as γ3. InFIG. 4 to FIGS. 6, γ1>γ2>γ3, and γ2=α2. The angle γ is appropriately setfrom the range of 10° to 70°.

As shown in FIG. 2, the stepped portion 24 is nearest to the inside ofthe main body portion in the regions corresponding to the centers of thearcuate major cutting edges 51. The stepped portion 24 is farthest fromthe inside of the main body portion in the regions corresponding to bothends of the plurality of arcuate major cutting edges 51. The arcuatemajor cutting edges 51 are used as major cutting edges. In the center ofeach arcuate major cutting edge 51, a large region functioning as a rakein the upper surface 2 can be secured. Therefore, the stability of thedirection in which generated chips are ejected can be improved.

As shown in FIG. 2, the upper surface 2 has a screw contact portion 8around the through-hole 7. The screw contact portion 8 comes intocontact with the screw head of an attaching screw, and has the sameshape as the contact surface of the attaching screw. When the attachingscrew is attached to the through-hole 7, the screw contact portion 8 andthe contact surface of the screw head come into close contact with eachother. With this configuration, the formation of a gap between the screwhead and the upper surface 2 is prevented, and chips are prevented fromsticking in the gap.

In this case, it is preferable that the screw contact portion 8 belocated at the same height as the cutting edge 5 or located at aposition lower than the cutting edge 5. In this example, as shown inFIG. 4 to FIG. 6, the screw contact portion 8 is located at a positionlower than the cutting edge 5. With this configuration, the positionwhere the screw head of the attaching screw and the screw contactportion 8 are in contact with each other is lower than the cutting edge5, and therefore chips are less likely to be stuck between the screwhead of the attaching screw and the screw contact portion 8.

As shown in FIG. 1, the angle between the upper surface 2 and the sidesurface 4 becomes larger as moving from the arcuate major cutting edge51 toward the arcuate minor cutting edge 52. That is, the clearanceangle becomes larger as moving from the arcuate major cutting edge 51toward the arcuate minor cutting edge 52. With this configuration, thecutting edge strength can be easily kept balanced and chatter vibrationcan be prevented by changing the clearance angle in accordance with thecutting force to which the cutting edge 5 is subjected. In this example,as shown in FIG. 4, the clearance angle is 0° in a cross section in thenormal direction of the center of each arcuate major cutting edge 51. Asshown in FIG. 7, the clearance angle is an acute angle in a crosssection in the normal direction of the center of each arcuate minorcutting edge 52.

Here, as shown in FIG. 4 to FIG. 9, in side view, clearance angle isinclination angle θ1, θ2 of the side surface 4 with respect to auxiliaryline V1, V2 perpendicular to reference plane h1, h2, respectively.Although clearance angle θ1, θ2 is not particularly limited, it ispreferably within a range of 0° to 15°, and may be appropriately setwithin this range. Specifically, in this example, clearance anglechanges smoothly from the arcuate major cutting edge 51 toward thearcuate minor cutting edge 52 such that θ1 in FIG. 5=0° and θ2 in FIG.8=8°.

The side surface 4 has recessed constraining portions. The constrainingportions include first constraining portions 41 located under thearcuate major cutting edges 51, and second constraining portions 42located under the arcuate minor cutting edges 52. The first constrainingportions 41 are inclined toward the inside of the main body portion 10as moving from the lower surface 3 toward the upper surface 2. Thesecond constraining portions 42 are inclined toward the outside of themain body portion 10 as moving from the lower surface 3 toward the uppersurface 2. With this configuration, the arcuate major cutting edges 51can be indexed using the first constraining portions 41. Therefore, thearcuate minor cutting edges 52 can be accurately indexed using thesecond constraining portions 42, and the insert 1 is less likely to bedetached from the holder.

In this example, as shown in FIG. 3, in side view, the firstconstraining portions 41 are located under the arcuate major cuttingedges 51. In side view, the second constraining portions 42 are locatedunder the arcuate minor cutting edges 52. With this configuration, thearcuate major cutting edges 51 and the arcuate minor cutting edges 52can be accurately indexed and can be used for cutting.

In this example, as shown in FIG. 4, the first constraining portions 41are inclined so as to approach the rotation center axis S1 as movingfrom the lower surface 3 toward the upper surface 2. As shown in FIG. 7,the second constraining portions 42 are inclined away from the rotationcenter axis S1 as moving from the lower surface 3 toward the uppersurface 2. With this configuration, the force to which the firstconstraining portions 41 are subjected from the holder in contacttherewith is in a direction from the upper surface 2 toward the lowersurface 3, and therefore the insert 1 is less likely to be detached fromthe holder.

Specifically, as shown in FIG. 4, the value of the inclination angle 6of the first constraining portions 41 with respect to a line I1 parallelto the rotation center axis S1, and the value of the inclination angle φof the second constraining portions 42 with respect to a line I2parallel to the rotation center axis S1 are both preferably 2° to 15°.Furthermore, as in this example, the value of the inclination angle δand the value of the inclination angle φ are preferably equal to eachother. With this configuration, the upper surface 2 of the insert 1 canbe used as a lower surface 3, and the lower surface 3 can be used as anupper surface 2, and therefore the economical efficiency is improved.

Furthermore, in this example, as shown in FIG. 10, the firstconstraining portions 41 and the second constraining portions 42 arearranged so as to form a regular hexagon centered at the rotation centeraxis S1. With this configuration, the rotational moment in the firstconstraining portions 41 is equal to the rotational moment in the secondconstraining portions 42, and the rotation of the insert 1 can beprevented.

In this example, from the viewpoint of maintaining the strength of theinsert 1, in order to smoothly connect the first constraining portions41 and the second constraining portions 42, continuous surfaces 43 areprovided. In this example, the first constraining portions 41 arelocated under the centers of the arcuate major cutting edges 51, and thecontinuous surfaces 43 are located under the ends of the arcuate majorcutting edges 51. As shown in FIG. 4 and FIG. 7, the inclination angleof the continuous surfaces 43 with respect to the rotation center axisis not particularly limited.

When the insert 1 is attached to the holder, the lower surface 3functions as a surface for contact with the holder. Therefore, the shapeof the lower surface 3 is not particularly limited as long as the lowersurface 3 is perpendicular to the rotation center axis S1.

In this example, the lower surface 3 has a shape obtained by rotatingthe upper surface 2 180 degrees about a straight line perpendicular tothe rotation center axis S1. That is, as shown in FIG. 11, as with theupper surface 2, the lower surface 3 includes first rake portions 31,second rake portions 32, a flat surface 33, and a stepped portion 34.With this configuration, the insert 1 can be used as a so-calleddouble-sided insert.

As shown in FIG. 11, the main body portion 10 has a lower cutting edge5U at the intersection of the lower surface 3 and the side surface 4.The lower cutting edge 5U includes arcuate lower major cutting edges 51Uthat are convex toward the outside of the main body portion 10, andarcuate lower minor cutting edges 52U that are convex toward the outsideof the main body portion 10. The curvature radius r2 of the arcuatelower minor cutting edges 52U is smaller than the curvature radius r1 ofthe arcuate lower major cutting edges 51U. In this example, as shown inFIG. 11, the cutting edge 5U has three cutting portions (50Ua, 50Ub,50Uc) each including one arcuate lower major cutting edge 51U and onearcuate lower minor cutting edge 52U. That is, the lower cutting edge 5Uhas three arcuate lower major cutting edges 51U (51Ua, 51Ub, 51Uc) andthree arcuate lower minor cutting edges 52U (52Ua, 52Ub, 52Uc).Therefore, the insert 1 is a six-corner type insert.

The curvature radius of the arcuate minor cutting edges 52 is smallerthan the curvature radius of the arcuate lower major cutting edges 51U.Therefore, even when the axial rake is negative, the arcuate lower majorcutting edges 51U are prevented from interfering with the workpiece.Therefore, sinking machining in which the cutting edge 5 moves in boththe direction parallel to the surface of the workpiece and the directionperpendicular thereto and performs cutting can be well performed.

The flat surface 33 in the lower surface 3 serves as a support surfacewhen the insert 1 is in contact with the holder. In this example, theflat surface 33 in the lower surface 3 is nearest to the arcuate lowerminor cutting edges 52U. With this configuration, a large flat surface33 can be secured in the lower surface 3 when the insert 1 is attachedto the holder. Therefore, the cutting force to which the arcuate majorcutting edges 51 are subjected can be received, and cutting constrainingforce can be improved and chatter vibration can be prevented.

<Cutting Holder>

As shown in FIG. 12, a cutting holder 100 that is an example of anembodiment of the present invention has insert pockets 101 formed in theouter peripheral surface of a cylindrical support main body portionhaving a rotation center axis S2.

As shown in FIG. 13, the insert pockets 101 each have a seating surface103 facing the rotational direction, and a plurality of constrainingside surfaces 104 located in directions intersecting with the seatingsurface 103. The plurality of constraining side surfaces 104 are eachinclined so as to approach a perpendicular of the seating surface 103with increasing distance from the seating surface 103. The seatingsurface 103 is provided with a screw hole 105 into which an attachingscrew (not shown) is screwed when the insert 1 is attached to the holder100. The seating surface 103 is in contact with the flat surface 23 ofthe insert 1. The plurality of constraining side surfaces 104 are incontact with the first constraining portions 41 of the insert 1.

With this configuration, the constraining side surfaces 104 are inclinedso as to approach a perpendicular I3 of the seating surface 103 withincreasing distance from the seating surface 103. Therefore, theconstraining surfaces 104 are less likely to be subjected to theresistance force due to screwing the attaching screw into the insert 1when the insert 1 is constrained. Therefore, the lower surface 3 of theinsert 1 and the seating surface 103 come into contact with each othermore easily, and the insert 1 is constrained stably. In this example,the inclination angle of the constraining side surfaces 104 are equal tothe inclination angle of the first constraining portions 41. With thisconfiguration, the constraining side surfaces 104 and the firstconstraining portions 41 come into contact with each other more surely.

Further, as shown in FIG. 13, the plurality of constraining sidesurfaces 104 are arranged so as to intersect with each other in planview. With this configuration, even in the case of machining in whichthrust force is large, such as machining of a difficult-to-cut material,the rotation of the insert can be prevented.

In this example, in order to maintain the stability of the chip ejectiondirection, chip ejection pockets 102 are provided. As shown in FIG. 12,from the viewpoint of improving the stability of the chip ejectiondirection, the chip ejection pockets 102 are preferably formed so as tobe continuous with the rear ends of the insert pockets 101.

<Cutting Tool>

As shown in FIG. 14, a cutting tool 110 that is an example of anembodiment of the present invention includes a cutting holder 100, andcutting inserts 1 attached to insert pockets 101. The cutting inserts 1are attached to the holder 100 such that arcuate major cutting edges 51protrude from the outer peripheral surface of the holder 100. That is,with this configuration, the arcuate major cutting edges 51 areconstrained at positions that are most protruded from the outerperipheral surface of the holder 100 at the time of cutting, and theeffect of the insert 1 of the present invention can be well exhibited.

In this example, as shown in FIG. 14, the inserts 1 are attached to theinsert pockets 101 with attaching screws 111. That is, each insert 1 isattached to the holder 100 by inserting an attaching screw 111 into thethrough-hole 7 of the insert 1, inserting the tip of the attaching screw111 into a screw hole formed in one of the insert pockets 101, andengaging the threads (not shown) with each other.

In this example, as shown in FIG. 14 (c), the inserts 1 are attached tothe holder 100 such that the arcuate major cutting edges 51 have apositive axial rake Ψ with respect to the rotation center axis S2 of theholder 100. That is, as shown in FIG. 14 (c), in side view, the cuttingedges 5 of the inserts 1 disposed on the outer peripheral surface sideof the holder 100 are inclined away from the rotation center axis S2 ofthe holder 100 from the front end of the holder 100 toward the rear endthereof. With this configuration, the cutting resistance applied at thetime of cutting can be reduced, and chatter vibration can be prevented.As a result, the fracture of the cutting edges 5 can be prevented.

<Method Of Manufacturing Machined Product>

With reference to FIG. 15, a method of manufacturing a machined productthat is an example of an embodiment of the present invention will bedescribed, by taking the case of using the cutting tool 10. The methodof manufacturing a machined product of this example includes thefollowing steps (i) to (iii):

-   (i) the step of rotating the cutting tool 110 in the direction of    arrow X about the rotation center axis S2 of the holder 100 as shown    in FIG. 15 (a), and the step of moving the cutting edges 5 of the    cutting tool 110 nearer to a workpiece 200 by moving the cutting    tool 110 in the direction of arrow Y1;-   (ii) the step of bringing the cutting edges 5 of the inserts 1 into    contact with the surface of the workpiece 200, moving the rotating    cutting tool 110, for example, in the direction of arrow Z, and    cutting the surface of the workpiece 200 as shown in FIG. 15 (b);    and-   (iii) the step of moving the rotating cutting tool 110 in the    direction of arrow Y2, and thereby separating the cutting tool 110    from the workpiece 200 as shown in FIG. 15 (c).

By cutting the workpiece 200 using the cutting tool 110 as describedabove, a machined product is manufactured.

In the step (i), the cutting tool 110 and the workpiece 200 merely needto be moved relatively nearer to each other. For example, the workpiece200 may be moved nearer to the cutting tool 110. Similarly, in the step(iii), the workpiece 200 and the cutting tool 110 merely need to bemoved relatively away from each other. For example, the workpiece 200may be moved away from the cutting tool 110. When the cutting iscontinued, the step of bringing the cutting edges 5 of the inserts 1into contact with a different location of the workpiece 200 may berepeated while maintaining a state where the cutting tool 110 is beingrotated. In this example, the cutting edge 5 of each insert 1 has threecutting portions as described above. Therefore, when the cutting portion(cutting edge 5) in use is worn, the insert 1 is rotated about therotation center axis S1 of the through-hole 7, and an unused cuttingportion (cutting edge 5) is used.

Typical examples of the material of the workpiece 200 include carbonsteel, alloy steel, stainless steel, cast iron, and nonferrous metal.

REFERENCE SIGNS LIST

1 cutting insert (insert)

2 upper surface

20 rake surface

21 first rake portion

22 second rake portion

23 flat surface

24 stepped portion

3 lower surface

30 rake surface on lower surface side

31 first rake portion on lower surface side

32 second rake portion on lower surface side

33 flat surface on lower surface side

34 stepped portion on lower surface side

4 side surface

41 first constraining portion

42 second constraining portion

43 continuous surface

5 cutting edge

50 cutting portion

51 arcuate major cutting edge

52 arcuate minor cutting edge

53 highest point

54 lowest point

5U lower cutting edge

50U cutting portion on lower cutting edge side

51U arcuate lower major cutting edge

52U arcuate lower minor cutting edge

53U highest point on lower cutting edge side

54U lowest point on lower cutting edge side

6 land

7 through-hole

8 screw contact portion

10 main body portion

100 holder

101 insert pocket

102 chip ejection pocket

103 seating surface

104 constraining side surface

105 screw hole

110 cutting tool

111 attaching screw

200 workpiece

1. A cutting insert comprising: a main body portion comprising: an uppersurface; a lower surface; a side surface connected to the upper surfaceand the lower surface; and a cutting edge located at an intersection ofthe upper surface and the side surface; a first rake portion on theupper surface; wherein the cutting edge comprises: an arcuate majorcutting edge that is convex toward outside of the main body portion; andan arcuate minor cutting edge that is convex toward outside of the mainbody portion and that has a curvature radius smaller than a curvatureradius of the arcuate major cutting edge, wherein the first rake portionis inclined to extend toward the lower surface as viewed in a directionfrom the arcuate major cutting edge toward an inside of the main bodyportion and is located along the arcuate major cutting edge, and whereinan inclination angle of the first rake portion decreases as viewed in adirection from a part continuous with one end of the arcuate majorcutting edge toward a part continuous with center of the arcuate majorcutting edge.
 2. The cutting insert according to claim 1, wherein thearcuate major cutting edge is inclined so as to approach the lowersurface with increasing distance from the arcuate minor cutting edge,and the arcuate minor cutting edge is inclined so as to approach thelower surface with increasing distance from the arcuate major cuttingedge.
 3. The cutting insert according to claim 1 wherein a highest pointof the cutting edge that is farthest from the lower surface is locatedbetween the arcuate major cutting edge and the arcuate minor cuttingedge.
 4. The cutting insert according to claim 1, wherein, in side view,the arcuate major cutting edge and the arcuate minor cutting edge arecurved toward a side away from the lower surface.
 5. The cutting insertaccording to claim 1, wherein the inclination angle of the first rakeportion decreases along a path from the part continuous with one end ofthe arcuate major cutting edge toward a part continuous with the otherend of the arcuate major cutting edge.
 6. The cutting insert accordingto claim 1, wherein, in plan view, the first rake portion is closer tothe inside of the main body portion in a region corresponding to thecenter of the arcuate major cutting edge than in regions correspondingto both ends of the arcuate major cutting edge.
 7. The cutting insertaccording to claim 1, wherein, in plan view, the arcuate major cuttingedge is longer than the arcuate minor cutting edge.
 8. The cuttinginsert according to claim 1 , wherein the upper surface comprises, alongthe arcuate minor cutting edge, a second rake portion that is inclinedto extend towards the lower surface as as viewed in a direction from thearcuate minor cutting edge toward the inside of the main body portion,and wherein an inclination angle of the second rake portion becomessmaller decreases along a path from a part continuous with one end ofthe arcuate minor cutting edge toward a part continuous with the centerof the arcuate minor cutting edge.
 9. The cutting insert according toclaim 8, wherein the inclination angle of the second rake portionbecomes smaller from the part continuous with one end of the arcuateminor cutting edge toward the part continuous with the other end of thearcuate major cutting edge.
 10. The cutting insert according to claim 8,wherein, in plan view, a width of the second rake portion as viewed in adirection from the arcuate minor cutting edge to the inside of the mainbody portion is substantially constant.
 11. The cutting insert accordingto claim 1, wherein, in side view, the side surface comprises a recessedconstraining portion, wherein the constraining portion comprises: afirst constraining portion located under the arcuate major cutting edge;and a second constraining portion located under the arcuate minorcutting edge, and wherein the first constraining portion is inclinedtoward the inside of the main body portion as viewed in a direction fromthe lower surface toward the upper surface, and the second constrainingportion is inclined toward the outside of the main body portion asviewed in a direction from the lower surface toward the upper surface.12. The cutting insert according to claim 1, wherein the main bodyportion comprises a through-hole penetrating the upper surface and thelower surface, and wherein the cutting edge comprises a plurality ofcutting portions each of which comprising the arcuate major cutting edgeand the arcuate minor cutting edge around the through-hole.
 13. Acutting tool comprising: the cutting insert according to claim 1; and acylindrical holder having a center rotational axis, wherein the cuttinginsert is attached to the holder such that the arcuate major cuttingedge protrudes from an outer peripheral surface of the holder.
 14. Thecutting tool according to claim 13, wherein the cutting insert isattached to the holder such that the arcuate major cutting edge has apositive axial rake with respect to the center rotational axis of theholder.
 15. A method of manufacturing a machined product, the methodcomprising steps of: rotating the cutting tool according to claim 13;bringing the cutting edge of the rotating cutting tool into contact witha workpiece and cutting the workpiece; and separating the cutting edgeof the rotating cutting tool from the workpiece.